The present invention relates to overload protection in a power transmission system using an endless member for transmitting power from one rotating member to another and, in particular, to overload protection in a variable speed V-belt drive of the type commonly used in mobile harvesting machines.
The mode of operation of variable speed V-belt drives is well known. Typically, power is transmitted between two shafts by means of drive and driven sheaves, both of which are made up of two halves axially adjustable in relation to each other, the axial adjustment changing the effective diameter of the sheave. The drive sheave may be adjusted hydraulically while the driven sheave is provided with spring means biasing the driven sheave halves in the direction of minimizing the space between them. Assuming the drive and driven shafts are at a fixed center distance, then adjusting the drive sheave to change the speed of the driven shaft will result in a compensating change in the driven sheave. For example, forcing the drive sheave halves together hydraulically so as to increase the effective diameter of the drive sheave will force the belt in the groove of the drive sheave outwards, momentarily increasing belt tension, and forcing the halves of the driven sheave apart, reducing its effective diameter until a new equilibrium condition is reached in which the driven shaft is turning faster than before and the drive belt is maintained at a tension sufficient for reasonable efficiency of drive transmission as a result of the spring force holding the halves of the driven sheave together.
It is well known to provide in the hydraulic control system for such a drive sheave, a pressure relief valve so as to limit the power which may be transmitted by the system. A variable speed V-belt drive with such a relief valve is described in U.S. Pat. No. 3,600,960, Karig. However, in many applications of such drives, a simple relief valve in the hydraulic system does not provide satisfactory protection against over-loading of the drive system. For example, in drives subject to widely fluctuating loading and/or the possibility of actually arresting or stopping the driven shaft but having only a power limiting hydraulic relief valve as described above, the relief valve essentially only determines the maximum belt tension which can be developed in the drive. Loading the drive beyond this power limit results merely in slipping of the drive belt with probable damage to the drive belt and possible fire risk from over-heating.